Theoretical Max

[trebuchet03]

I want to bounce this question around... It might prove useful when shopping for cars and cost v benefit analysis.

How to determine the maximum fuel efficiency of a particular engine/transmission/car set.

I ask this question because when I enter my neighborhood, I try to keep it in 5th with no throttle input - just let it idle right around 19 or so mph. I get ~50mpg on the SG while doing so.


So in that condition - throttle input is at it's lowest (in my case, the same clutch in or out) and rpm's were at the theoretical minimum (engine idle).

Vitals on a flat, no wind, fully warm
TPS: 3 (out of 75)
MPG: 50.0
MPH: ~15
RPM: ~800
Road: Asphalt

If anyone else would like to post what their car gets, that'd be great

So obviously speed isn't exactly linear to throttle position. I'm curious if there's any existing methodology to measure this, or, if it's simply a moot point

[Frank Lee]

Ca + Cm + Cf + Ci + Cl/My = Cml

Where:

Ca = Cost to Acquire Vehicle
Cm = Cost of Maintenance
Cf = Cost of Fuel
Ci = Cost of Insurance
Cl = Cost of License
My = Miles Driven per Year
Cml = Cost per Mile
********************
Whoops! That's the main equation I care about.

If I understand this right, you are wanting to know:

FEmax @ TImin when in condition Gtop, where:

FEmax = Maximum Fuel Efficiency
TImin = Minimum Throttle Input
Gtop = Top Gear

To that I would ask: What percentage of time does the vehicle operate in FEmax? My guess is <1%, and I think <1% = moot.

[bbjsw10]

Wow, Frank didn't know you had it in ya.

trebuchet03, I may have to try this. I finally got my MpGuino going. I am sure you are probably not in the market for an Xfi though. I am afraid to bog it too low though. 3cyl=no torque.

[trebuchet03]

Originally Posted by Frank Lee

If I understand this right, you are wanting to know:

FEmax @ TImin when in condition Gtop, where:

FEmax = Maximum Fuel Efficiency
TImin = Minimum Throttle Input
Gtop = Top Gear

Not so much - I want to know FEmax @ Xcondition. Whatever that condition is. For me, the highest FE I've been able to get happens to be in the conditino I described in my first post.

For example.... You get 24mpg in your car... but the maximum attainable is 40. That means you're operating at 60% of maximum fuel efficiency. Now if you're getting 38mpg and your max is 40 - you're at 95%. And the closer you are to 100%, the more difficult it is to increase.

But, that said, the cml equation is a good thing for the back pocket (thanks)

[Frank Lee]

"TImin when in condition Gtop"

-isn't that the condition where every vehicle gets FEmax?

(and, I had considered "FEmax @ TImin when in condition Gtop = x, but decided that wasn't it.)

[trebuchet03]

Originally Posted by Frank Lee

"TImin when in condition Gtop"

-isn't that the condition where every vehicle gets FEmax?

Maybe I'm just misunderstanding you (or not explaining myself properly)....

Minimum throttle would be idle throttle. Top gear isn't variable - so that would mean the condition is idling in gear. But, I think I'll make the claim that this isn't always true. If a small throttle input results in a much higher (disproportionate) speed - the minimum throttle position wouldn't be where theoretical maximum FE point is.

The more I think about this, the more I come to the conclusion that there is no trivial solution.

[cfg83]

trebuchet03 -

I have lots of roads "on the flat" where I idle in gear. I will get back to you. I think the LOD parameter and engine displacement is also relevant.

CarloSW2

[tasdrouille]

Most cars get their best fuel economy in top gear in the 35-45 mph range (From an SAE paper I read). There usually is a significant efficiency penalty at low throttle and rpm.

[trebuchet03]

Originally Posted by tasdrouille

Most cars get their best fuel economy in top gear in the 35-45 mph range (From an SAE paper I read). There usually is a significant efficiency penalty at low throttle and rpm.

So I'm trying to find more combinations with that FE or greater... I found this today


TPS: 8 (out of 75)
MPG: 50.0
MPH: 33
RPM: ~1695
Road: Asphalt

I wasn't using CC, and that was an instantaneous read (I'll have to retest under better conditions)... But, it makes me want to create an FE map -> throttle v. mpg v. speed

[bikin' Ed]

Wouldn't infinity be your theoretical max mpg? i.e. going down hill during EOC. Trying to factor in grade, road surface, weight of vehicle (how much fuel, how many passengers, cargo) air speed, wind speed and angle and whatever else I missed, how can hope to get even a formula for max mpg?